Polyorganosiloxanes, i.e. polymers having a backbone consisting of alternating silicon and oxygen atoms and pendant organic groups attached to the silicon atoms of the backbone, are well-known and appreciated for their unique combination of properties including low glass transition temperatures, low surface tension, weathering resistance, and biocompatibility. These materials exhibit generally remarkable chemical and thermal stabilities, however, the polar nature of the siloxane bond and the high flexibility of the polymer backbone render them prone to degradation by ionic substances such as acids or bases, in particular at high temperatures. Carbosiloxane polymers, sometimes also referred to as hybrid silicones, i.e. polymers having a backbone comprising besides oxygen atoms also organic moieties such as alkylene, arylene or fluoroalkylene groups as groups bridging adjacent Si atoms, are less susceptible to molecular chain scission by ionic substances, which has been attributed to the less polar nature of the Si—C bond and a stiffening of the backbone by implementation of the organic groups preventing spatial reorganization to a cyclic transition state associated with degradation mechanisms. Thus carbosiloxane polymers such as polysilalkylenesiloxanes represent an attractive alternative to polyorganosiloxane materials in applications where thermal and chemical stability under demanding conditions is required.
A variety of different processes for the production of linear carbosiloxane polymers has been developed in the past. In particular polysilalkylenesiloxanes have been prepared by three principal synthetic routes, namely A) by polycondensation of bis-silanol terminated silalkylene precursors, B) by hydrosilylation polymerization of an α,ω-diene and an α,ω-dihydrodisiloxane, and C) by ring opening polymerization of cyclic silalkylenesiloxane monomers.
Step growth condensation polymerization of bis-silanol terminated silalkylene compounds yields polysilalkylenesiloxanes of merely moderate molecular weight (Mn of less than 50,000 g/mol), see for example U.S. Pat. No. 5,386,049 and A. Benouargha et al., Eur. Polym. J., 33 (1997), 1117-1124. The obtained product has silanol end groups. A terminal functionalization of the product such as for example by introducing vinyl-functional end groups requires an additional synthetic step.
Also the polymerization by means of hydrosilylation step growth has several inherent disadvantages: Due to the nature of the reactants the organic bridging groups in the resulting carbosiloxane polymer necessarily have at least two carbon atoms. Thus in particular polysilmethylenesiloxanes cannot be synthesized by the hydrosilylation route. Moreover, the reaction is very sensitive to the stoichiometric balance of the reactants. Side reactions which cause deviations from this balance limit the obtainable degree of polymerization. For instance, transition metal catalysts typically used for hydrosilylation reactions can isomerize a terminal carbon-carbon double bond to an internal position, thereby converting the α,ω-diene into a monofunctional species. Consequently typically only polysilalkylenesiloxanes of low molecular masses (Mn<30,000 g/mol) are obtainable by the hydrosilylation route.
Despite of the limitations with regards to the obtainable degree of polymerization, the hydrosilylation technique has been found valuable in the preparation of cyclic monomers that can be used to synthesize polysilalkylenesiloxanes by ring opening polymerization. Thus Tapsak et al., Journal of Inorganic and Organometallic Polymers, 9 (1) (1999), 35-53 describe the preparation of cyclosilalkylenesiloxane monomers containing alkylene units having from 6 to 14 carbon atoms by synthesizing in a first step polysilalkylenesiloxanes of relatively low molecular weight by hydrosilylation from 1,1,3,3-tetramethyldisiloxane and 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene or 1,13-tetradecadiene, respectively. These polymers were then subjected to depolymerization in presence of KOH at high temperature (200° C.) to obtain a mixture of cyclic monomers, separated by means of vacuum distillation. The cyclic silalkylenesiloxane monomers were finally purified by recrystallization or vacuum distillation and subsequently subjected to ring opening polymerization using an acidic catalyst (Dowex 50 W ion exchange resin) to obtain polysilalkylene-siloxanes of significantly higher molecular mass (Mn up to 346,000 g/mol) than achievable by the hydrosilylation route. This reference thus provides a process enabling to obtain polysilalkylenesiloxanes having a comparatively high molecular weight (Mn>100,000 g/mol), however, no comprehensive control and engineering of the molecular weight was demonstrated. Moreover the ring opening polymerization according to this process requires quite stringent reaction conditions involving reaction temperatures of about 140° C. Polysilalkylene-siloxanes comprising methylene bridging units cannot be obtained with this method since the unsaturated precursor used in the hydrosilylation step necessarily contains terminal carbon-carbon double bonds and thereby yields alkylene units having at least two carbon atoms.
U.S. Pat. No. 6,492,480 describes a method for the preparation of a linear polysilalkylenesiloxane by subjecting a four- to seven-membered cyclic silalkylene monomer including also silmethylenesiloxane to ring opening polymerization in a non-aqueous solvent under mild and neutral conditions using a specific type of polynuclear ruthenium-carbonyl complex as catalyst in the presence of a silane compound having at least one Si—H bond as co-catalyst. Typically the equilibrium between the desired high molecular linear polysilalkylenesiloxane, linear and cyclic oligomers and unused monomer is reached within a reasonable reaction time of a few hours at a reaction temperature in the range from 30 to 50° C. The specific catalyst/co-catalyst system enables a control of the molecular weight of the resulting polymer without use of a chain transfer agent by simple variation of the concentration of the monomer, the catalyst and the silane compound as well as the choice of the solvent. The resulting polymer can have a considerably high degree of polymerization (Mn up to 157,000 g/mol) and generally has a narrow molecular weight distribution with a polydispersity index in the range from 1.3 to 2.6. However, this process requires the use of an expensive complex transition metal catalyst. Additionally, removal of the solvent after the completion of the reaction is necessary. Moreover, while controlled variation of the molecular weight may be possible in a wide range, this process itself provides only very limited means to control the end groups as the resulting polymer usually has a hydrogen atom or, when water is present in the reaction system, a hydroxyl group, at each end. Thus further functionalization would require an additional process step.
Interrante et. al describe the preparation of poly(dimethylsilylenemethylene-co-dimethylsiloxane) by ring opening polymerization of 1,1,3,3,5,5,7,7-octamethyl-2,6-dioxa-1,3,5,7-tetrasilacyclooctane in the presence of triflic acid as catalyst at room temperature (Macromolecules, 34 (2001), 1545-1547). The used monomer was obtained by reacting ClSi(Me)2CH2Si(Me)2Cl in ethyl acetate with zinc oxide and subsequent fractional vacuum distillation. Although the authors demonstrated that polysilmethylenesiloxanes could be obtained by this process with reasonable yield under mild conditions without the need for costly complex catalysts, they were only able to prepare polymers of moderate molecular weight (Mn of about 50,000 g/mol). The process also yielded a significant proportion of undesired macrocyclic molecules. Moreover, no means for a control of the molecular weight of the resulting polymer nor its end groups were provided.
U.S. Pat. No. 8,586,690 proposes a two-step process for the production of high molecular weight linear polysilalkylenesiloxanes. Initially a cyclic silalkylenesiloxane monomer is subjected to a first ring opening polymerization in the presence of a basic or acidic catalyst to form a mixture of siloxane monomers and linear oligomers. The oligomers are then precipitated by addition of a suitable solvent and removed from the mixture. The remaining siloxane monomer mixture is subsequently used as a starting material in a second ring opening polymerization step, which is to be conducted within the melting point range of the monomer mixture. It was shown that polysilalkylenesiloxane of significantly higher molecular weight (Mn above 100,000 g/mol) could be obtained by this two step process compared to direct single step ring opening polymerization of the initial cyclic monomer, however, the reason for this surprising finding remained unclear. Using 1,1,3,3,5,5,7,7-octamethyl-2,6-dioxa-1,3,5,7-tetrasilacyclooctane as starting cyclic monomer it was in particular demonstrated that also polysilmethylene-siloxane polymers could thus be obtained under mild reaction conditions using inexpensive readily available acidic or basic ring opening polymerization catalysts without the above-mentioned limitations to the molecular weight found by Interrante et al. However, the implementation of a second polymerization step increases the complexity of the preparation method. As a further disadvantage in view of process efficiency, a part of the starting material is wasted as the linear oligomers separated after the first ring opening polymerization step are discarded. Moreover, the process according to U.S. Pat. No. 8,586,690 includes no means allowing controlling the molecular weight and/or the end groups of the resulting polymer. Such control would however be helpful as it imparts the ability to engineer the properties of the resulting polymer material such as its rheological characteristics or its cross-linkability according to the requirements of the intended application.